The Trinity Complex (Northern California, U.S.A.) exposes numerous gabbro intrusions, up to 8 km across, within a mantle peridotite host. Their ages ranges from 404 to 431 Ma.. The juxtaposition of mantle and crust rendered the Trinity Complex a good analogue to slow spreading oceanic crust and mantle. Accordingly, three plutons were selected to perform a detailed geological study with the aim to enhance our understanding of how oceanic crust is constructed when plates slowly diverge. Mapping of the selected Trinity gabbros demonstrated that – in contrast to typical ophiolites – a regular stacked pseudo-stratigraphic is not present, mantle flow appears not related to pluton fabrics, a uniform extension direction is not obvious (i.e. dykes/local swarms have a diverse orientation and pluton cross sections are equant in map view), and tectonic extension, normally abundant during slow spreading conditions, is absent. In contrast, there exists overwhelming evidence for a stable and relatively cold lithospheric setting, e.g. cm- to meter-scale host-rock fragments lining contacts or marking completely disintegrated former cumulate strata, absent to poorly developed magmatic fabrics, juxtaposition of rocks which formed at widely different temperatures, i.e. pyroxenite in gabbro, dolerite in peridotite; and, finally, in the Northwestern Trinity Complex, gabbros intruding in a preexisting, structurally and geochemically distinct, Neoproterozoic crust. Three intrusive series 1-3 could be discriminated during field work. For all plutons, the order of crystallization is olivine, followed by pyroxene (typically cpx before opx), plagioclase and finally amphibole. Consequently, the appearance of cumulate rocks is dunite => wehrlite => pyroxenite => gabbro => plagiogranite. This order of crystallisation, and the extremely anorthite-rich nature of plagioclase (implying a water-rich magma) support an origin in a supra-subduction zone setting. Low concentrations of Na, Al, and Ti mark cpx as chemically distinct from that in oceanic gabbros. By considering mineral chemical data, the Rare Earth Elements (REE), and the High Field Strength Elements (HFSE), the mapped intrusive Paleozoic Series 1-3 can all be derived from a single parent magma by fractional crystallization only. Very low REE and HFSE contents for all series demonstrate their derivation from a depleted source. For the Large Ion Lithophile Element (LILE) content of the dolerites, however, the spread in the data exceeds the range explicable by fractionation alone such that different kinds of source magmas having proprietary LILE contents need to be called for. It is currently not clear whether these diverse LILE components can be correlated with the mapped gabbro series 1-3. Even though in the Trinity Complex spreading has been limited and magmatism has been long-lived (as indicated by age data) no laterally continuous crust was formed. This suggests that the magma budget – and thus also the heat budget – was low. This conclusion is consistent with generally observed cold magmatic contacts and a chemically depleted source capable of generating only limited amounts of melts. In order to explain field, mineralogic and geochemical data an origin in a weakly extensional fore arc environment appears the most suitable setting. Subduction probably involved a spreading ridge. This overall setting allowed for the abundant supply of water and LILE from the downgoing plate. The subducting ridge would guarantee a high heat flow and its low density would prevent a roll-back and thus prohibit massive fore-arc extension. The presence of Neoproterozoic crust (and mantle?) independently confirms that a successful rifting did not occur in the case of the Trinity Complex. It is concluded that the Trinity Complex appears poorly suited as an on-land analogue for slow spreading environments (such as in the Indian Ocean), instead, evidence points to a relatively stable, lithospheric fore-arc setting.